Commercially available ceramic matrix composites (CMCS) have many potential applications in high temperature environments. CMCs are somewhat limited in their capability for exposure to temperatures exceeding values in the order of approximately 1,200° C. In addition, CMCs typically cannot be cooled effectively under high heat flux conditions because of their relatively low thermal conductivity. Accordingly, it is known to provide a thermally insulating material for application to a ceramic matrix composite substrate material so that the insulated CMC composite can be used in a high temperature environment, such as the hot gas flow path of a gas turbine engine.
U.S. Pat. No. 6,197,424, assigned to the present assignee and incorporated herein by reference in its entirety, describes one example of a thermally insulating material for application to the ceramic matrix composite substrate. More specifically, the '424 patent describes a gas turbine component fabricated from CMC material and covered by a layer of a dimensionally stable, abradable, ceramic insulating material, commonly referred to as friable graded insulation (FGI).
Typically, the thermally insulating material may be susceptible to environmental effects, such as moisture attack, particulate-induced erosion, etc., that over time tend to reduce the thickness of the insulating material. Traditional inspection techniques have been generally unavailable for determining the thickness of the insulating material. For example, visual inspection has been of limited practical utility since the thermally insulating material and the underlying CMC material are essentially visually undistinguishable from one another. Similarly, electromagnetic sensing techniques, such as may employ eddy current sensors, are practically unusable in view of the relatively low conductive properties of the CMC and the thermally insulating material. Present techniques for determining the thickness of the insulating material generally involve time-consuming and burdensome tools, such as CMM or bench layout. Thus, there is a need for inspection techniques that may quickly and inexpensively allow evaluating the thickness of the thermal insulation over a ceramic matrix composite substrate material.